The aforementioned technical paper describes a microprocessor-based automatic draft control system for a three-point hitch. A tillage implement is attached to the hitch and the hitch is mounted at the rear of a tractor having pneumatic tires. A single quadrant lever, located in the tractor cab, is manually moved by the operator to generate position/draft commands which cause raising or lowering of the hitch. Draft sensors sense the draft on the hitch and a position sensor senses the position of the hitch, the sensors producing feedback signals to the microprocessor. The microprocessor responds to the position/draft commands and the feedback signals from the sensors by generating output signals for adjusting the hitch to the position commanded by the quadrant lever. The microprocessor output signals are applied to a solenoid of a closed-center valve, the valve in turn controlling hydraulic flow applied to a lift-cylinder which raises the hitch.
While systems of the type described by Macqueene et al. work quite well, uncommanded movement of the hitch may occur if a valve solenoid is energized through use of a low side driver. That is, if one end of the solenoid is connected to a positive voltage and the solenoid is energized by selectively connecting the other end of the solenoid to ground through a switch (driver) any inadvertent grounding of the wire lead connecting the switch to the solenoid or driver failure in an "on" state causes the solenoid to be energized. This, in turn, causes uncommanded movement of the hitch.
If the system is operating in a closed-loop mode, the feedback signals resulting from the uncommanded movement act through the feedback loop so that the hitch position commanded by the quadrant lever is maintained. However, the closed-loop mode may be aborted so that the operator may use fender mounted switches to control raising or lowering the hitch in an open-loop mode thereby facilitating attachment or removal of an implement from the hitch. Also, at start-up the hitch may not be under closed-loop control because the hitch is not "captured", i.e. at the position commanded by the quadrant lever. If the lead between the valve solenoid and its driver switch should be shorted to ground while the system is in an open-loop mode, uncommanded movement of the hitch will take place. Such movement, of course, could be dangerous.